James Ahiakpor asked:
>In what sense was the theory of demand and supply as an 
>explanation of market price determination a post-1850 development?


As with all things, it depends on precisely what you mean.  Classical
economists knew and used supply and demand, as Dennis O"Brien nicely
shows in his wonderful book revisiting classical economics. But their
focus was on supply forces and costs as determinants of value--demand
was a side issue, and given far less focus. Thus, they specifically said
that their major analysis did not cover non-reproducible goods where
demand forces were seen as central.  

With the introduction of marginal and  neoclassical ideas primarily in
the late 1800s, but which existed earlier in less well known work, the
focus shifted initially to demand, and then to supply and demand. That's
the story told in standard history of thought books, including Landreth's
and mine. 

So, in answer to James's question, the sense is one of focus of
analysis, not of understanding.

Dave Colander